On December 7th, 2018 at 1823 UTC the Chinese launched the much anticipated Chang’e 4 mission to the far side of the Moon. Unfortunately, the launch was not broadcast live nor has there been much in the way of official word on what is going on with the mission. Fortunately, we have both the Chang’e program history and Chang’e 4’s transmitter to help us fill in the blanks of what to expect and witness history in realtime without a filter.
So for those not accustomed to watching a space first unfold in the language of the Doppler effect here’s a primer of what has happened and what to expect.
The Doppler Effect, our way to the Light Side…
Anything emitting a wave moving relative to an observer will have the Doppler effect be observable in the received waves. So if the emitter is moving toward you the waves will get shorter and the pitch will increase. If the emitter is moving away then the waves appear longer and the pitch will decrease. The Doppler effect is a relative thing and the relative velocity between the emitter of the wave and the hearer of those waves sums together to produce the resulting effect.
So when we are listening to an object headed to the Moon that is emitting a radio signal that signal will be affected by the velocity of the spacecraft heading away from us and our motion as the Earth rotates, amoung other minor effects.
The same thing will happen once the spacecraft reaches the Moon. But the proportions of those relative velocities will change providing us a means to measure the orbital parameters and note things about the spacecraft’s trajectory.
In effect it’s like sitting in Chinese mission control, we don’t have all the details but if it goes right or wrong we will see an effect and can from that effect make judgements about how the mission is going.
So we are not in the dark, we can watch the mission unfold in real time without the need for the Chinese State Media to tell us what’s happening on a macro level.
A Trip to the Moon, Riding a Descending Tone…
I tried looking for the Tracking, Telemetry and Control (TT&C) beacon the day of launch but unfortunately the spacecraft was low in the sky and obscured by trees that greatly attenuate radio signals from deep space. I had an idea of where to look thanks to the handy work of fellow satellite tracking enthusiast Bill Gray. Bill published a series of orbital elements for the Chang’e 4 payload based on past Chang’e program history and I used those as a basis to begin my search.
Above is the expected Doppler behaviour of Chang’e 4 in its orbit to the moon and back assuming the moon wasn’t in the way. It illustrates the dynamics involved graphically. The red portions are the predicted Doppler effect on the emitted radio beacon onboard Chang’e 4 as heard at my station. The white dots are signal actually heard so far. The ‘sine wavy’ part is introduced as a result of Earth’s rotation. The ever ascending ‘offset’ is the part imparted by the motion of the spacecraft in it’s orbit.
The following day, I was able to obtain acquisition of signal of the TT&C beacon on about 2210.81MHz and even noted her sister ship already in lunar orbit, the service module from the Chang’e 5-T1 test mission launched in 2014. What is Chang’e 5T-1?
Day 1 – First signals from Chang’e 4 detected along with Sister ship Chang’e 5-T1 presently in a 240km high lunar orbit.
The signal was weak but well within a usable signal to noise ratio to allow for Doppler analysis. Comparing the signal to the expected orbital elements Bill had published resulted in a good fit. Only minor corrections to Mean Anomaly where required to get me bang on target and peak the signal in the tracking antenna.
Day 1 – Doppler curve reveals probable evidence of a maneuver despite Chinese State Media claims one was not needed.
Interestingly, the signal revealed possible evidence of a maneuver by way of a pronounced deviation from the excepted trend in the Doppler data on day 1 of the observations. This deviation in expected Doppler behaviour began around 21:47 UTC on Dec 8th. Chinese state media outlets made statements that the maneuver was not required at 11:42 UTC on the 8th. Chinese Space watcher @LiuyiYiliu on twitter provided different timings for the maneuvers. So what happened? I’m not sure.
Also of value in the post above is the trajectory and apparently the state vector the Chang’e 3 lander mission. Note the orbit Chang’e 3 entered upon arrival at the Moon.
Day 2 tracking began and ended as expected with no pronounced deviation from the expected mission. No evidence of significant deviation from predicted Doppler was observed.
Day 2 – Observed Doppler behaviour indicating the mission appears on track.
Day 3 of the mission arrived with acquisition of signal on time and with levels as expected. As Chang’e 4 approaches the moon my antenna’s beam is within range to catch whiffs of Chang’e 5-T1’s signal at times.
Day 3 – As Chang’e 4 approaches the moon it is greeted by Chang’e 5T-1 already in lunar orbit.
What to Expect Upon Arrival in Lunar Orbit
Sometime on Dec
11th 12th the spacecraft should perform a lunar injection burn to enter orbit around the Moon. I would expect the spacecraft to first enter a polar orbit much like Chang’e 3 did. From there the lander stack should maneuver again to enter an orbit in line with the latitude of the landing site at the Von Karman crater on the far side of the moon whose latitude is 44.5 degrees south.
The chosen landing place for Chang’e 4, Van Karram crater complex at 45 degrees south latitude.
This is interesting as Chang’e 5-T1 last phase of it’s mission was to test the landing approach for the upcoming Chang’e 5 sample return mission at Mons Rumker which is at 41 degrees north as recently publicly claimed in a 2017 Chinese State Media post. The mission profile for Chang’e 3 was also very similar with a landing site at 44 degrees latitude at Sinus Iridum.
This coincidence provides some credibility that the Chinese will use a similar procedural process to inject Chang’e 4 into a similar orbit. It only stands to reason they would want to use procedures they know to lower the risk. All the evidence of the landing site latitude selected seems to point toward this outcome.
The chosen place for the Chang’e 5 sample return mission, Mons Rumker at 41 degrees north latitude.
So based on this evidence and the history of the Chinese Chang’e program of building upon previously tested techniques, I expect the first orbit obtained will resemble the current polar orbit of the NASA Lunar reconnaissance orbiter (LRO) and produce a Doppler curve as follows. Any deviation will be obvious. If they miss and end up in a highly eccentric orbit or worse don’t enter orbit at all it will be obvious in the Doppler data.
A full lunar day (28 Earth days) of Doppler Data of the NASA LRO mission in polar orbit around the moon. It shows the distinct characteristics of the plane of the orbit seen from a different angle from Earth as the Moon orbits the Earth.
You can tell that the LRO orbit is polar by the fortnightly sinusoidal changes in Doppler amplitude as the plane of the orbit becomes edge on to us on Earth and perpendicular ~14 days later. When edge on, the relative velocity observed here on Earth is at its greatest thus producing the greatest swings in Doppler pitch. Conversely when the plane is perpendicular to us there is a brief time when the entire orbit should become visible from Earth and there will be very little relative velocity as seen by us here and only the component of the rotation of the Earth will be visible.
Once in the polar orbit either a maneuver or series of maneuvers will take place to lower the inclination to the final decent orbit with an orbital inclination of the landing site. As night is descending over the landing site now they will need to wait and as a result have plenty of time before the next lunar day begins on the far side to execute a landing early next year. I expect the Doppler characteristics of the 45 degree inclined preliminary decent orbit to closely resemble that of the Chang’e 5-T1’s orbit as noted below at least before they start to elongate it during the decent procedure.
A lunar pass as seen from Earth of Chang’e 5T-1 Doppler behaviour. Note the pronounced effect of the orbital motion around the moon (the sinusoidal shape) and subtle effect of the Earth’s rotation with the offset of the curve changing downward each orbit as the Moon pass progresses.
Assuming the worst and a botched lunar injection occurs and they ended up in a highly elliptical orbit the Doppler curve could resemble something like that of DSLWP-B also presently active in lunar orbit.
The amateur radio lunar satellite DSLWP-B is in a highly elliptical lunar orbit which has a pronouncely different Doppler curve than a low lunar orbit satellite with a low eccentricity.
If the Lunar injection maneuver fails to happen the Moon’s gravity will slingshot Chang’e 4 around it and change its orbit around the Earth but it will return to Earth’s vicinity much as the upper rocket stage is predicted to. This should result in a Doppler curve resembling the curve that it produced on the way to the Moon to some degree.
A new post by @LiuyiYiliu provides some timings for a correction burn on Dec 11th at 08:42 UTC and the final Trans Lunar Injection (TLI) burn on Dec 12th at 08:46UTC. These burns will occur with the Moon as high as she can go over China.
Location of the Moon for the expected Trans-Lunar Injection burn on Dec 12th at 08:46 UTC.
Let’s hope we see a successful capture into the right orbit! So sit back and watch as the Doppler effect tells the tale of Chang’e 4’s mission to the Moon and illuminates the dark side of the Moon…
More posts will be added as the mission evolves so if your intrigued by my posts on Twitter check back here regularly for more details…